The present invention relates to an easily maintained bubble-cap column, especially for the distillation of clogging and high-sediment liquids.
The distillation of liquids which have a high sediment content and/or tend to clog the distillation column causes problems in several areas of industry. Perforated column plates are recommended for use in the distillation of such "foul" liquids. The more difficult the liquid is to handle--with a tendency to polymerize or burn onto the apparatus--the larger (up to 1/2") the perforations in the plates are made (Pyle, C., in Distillation in Practice, Ed. Nielsen, C. H., Reinhold Publishing Corp., New York 1956, pp. 13, 14; Hengstebeck, R. J., Distillation Principles and Design Procedures, Reinhold Publishing Corp., New York 1961, pp. 45-47). In special cases the diameter of the perforations in the plates can be made up to 2-3 cm, in which case the rest of the column structure must be modified accordingly (Billet, R., Industrielle Destillation, Verlag Chemie, Weinheim 1973, p. 333). During distillation, as the perforation size and the number of open perforations gradually decreases owing to clogging, the vapor consumption of the column increases and its separation capacity drops. In this case the column must be cleaned at some stage. The specific advantage of a perforated column plate is that it is relatively easy to clean owing to its simple structure. Perforated plates have, however, one significant drawback. The liquid under distillation does not remain on the plates but flows down if vapor pressure in the column drops sufficiently even for a moment (van Winkle, M., Distillation, McGraw-Hill Book Comp., New York 1967, p. 489), and thereby there is a risk of the vapor-liquid balance being disturbed in the column and of the distilled liquids passing into waste water through the floor of the column. Even though the latter possibility can be effectively prevented by current control techniques, even a short complete vapor cutoff in a perforated-plate column always causes a long stoppage in distillation as the liquids escape from the plates. The refilling of the column and the re-establishing of the vapor-liquid balance require both additional energy and time. Bubble-cap columns are significantly more stable in corresponding cases of vapor pressure disturbance, since in a correctly constructed bubble-cap column the liquid will not flow down when vapor pressure drops but will remain on the plates. The pressure having returned to normal, distillation can be continued after a rather short balancing. It is because of this improved stability--compared with perforated-plate columns--that bubble-cap columns are used in industry for the distillation of clogging liquids (Kretzschmar, H., Hefe and Alkohol sowie andere Garungsproduke, Springer-Verlag, Berlin, Gottingen, Heidelberg 1955, p. 435), even though bubble-cap plate structures are complicated and, after clogging, the job of cleaning them is time-consuming and the results of this cleaning are often moderate at best. The effectiveness of the cleaning can be observed from the pressure losses after the distillation apparatus has been restarted, the losses being clearly higher after unsuccessful cleaning than in a new, completely clean column of the same type. The higher the pressure losses after cleaning, the shorter will be the next distillation period before new cleaning. This causes many disadvantages, such as waste of energy (extra pressure losses require additional vaporization), a shortening of the effective operating period of the distillation unit, lowering of the capacity, and additional waste water load due to frequently repeated washes. In order to eliminate these drawbacks, the present invention relates to a more advanced bubble-cap column, in which the distillation periods of high-sediment and clogging liquids are many times longer than those of corresponding other bubble-cap columns used for the same purpose. In addition, the column can, when needed, be cleaned quickly and so thoroughly that the operating periods of the unit are not shortened in comparison with operating periods observed using a new and fully clean unit.
It is generally known that it is possible to eliminate or substantially diminish the sedimentation, on the inner surfaces of a reactor, of materials which are produced as a result of various chemical reactions and are insoluble in the reaction medium, by coating the inner surfaces of the reactor with polytetrafluoroethylene. Such coatings, however, wear off and therefore must be periodically renewed, which has proven to be very difficult and time-consuming.
Therefore the object of the present invention is to provide a bubble-cap column which is easier to maintain than previous ones, which can be used for longer periods without interruption for cleaning, and which is easier and quicker to clean.